package Task13;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

/**
 * 动态规划跳石板
 */

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int m = scanner.nextInt();
        int[] step =new int[m+1];
        for (int i = 0; i < m + 1; i++) {
            step[i] = Integer.MAX_VALUE;
        }

        step[n] = 0;
        for (int i = n; i < m; i++) {
            if(step[i] == Integer.MAX_VALUE){
                continue;
            }
            List<Integer> list = div(i);
            for (int j: list) {
                if(i + j <= m && step[i + j] != Integer.MAX_VALUE){
                    step[i+j] = Math.min(step[i+j],step[i]+1);
                } else if (i + j <= m) {
                    step[i+j] = step[i] +1;
                }
            }
        }
//        for (int i = 0; i < step.length; i++) {
//            System.out.println(step[i]);
//        }
        if(step[m] == Integer.MAX_VALUE){
            System.out.println(-1);
        }else{
            System.out.println(step[m]);
        }


    }
    public static List<Integer> div(int num){
        List<Integer> list = new ArrayList<>();
        for (int i = 2; i*i <= num; i++) {
            if(num % i == 0){
                list.add(i);
                if(num/i != i){
                    list.add(num/i);
                }
            }

        }
        return list;
    }
}
